Effect of copper impurities on the absorption spectrum of thin films of superionic conductors MAg4I5 (M = K, Rb)

The absorption spectra of thin films of the solid electrolytes MAg₄I₅ (M = K, Rb) doped with copper (0 ≤ x ≤ 0.15) are studied in the spectral range 2–6 eV at temperatures of 90 and 290 K. It is established that the critical Cu content reaches x _crit = 0.05. A decrease in the Cu content to x ≤ 0.05 leads to the formation of MAg_{4 ? 4x} Cu_4x I₅ solid solutions, and the films remain spectrally stable. At x > 0.05, the films segregate into different phases: MAg_{4 ? 4x} Cu_4x I₅, Ag_{1 ? x} Cu_xI, and M ₂AgI₃.